General structure
An amino acid contains an amino group, –NH2, and a carboxylic acid group, –COOH. In an α-amino acid, both groups are bonded to the same carbon atom.
The general formula of an α-amino acid is:
The R group is the variable side chain. Its structure distinguishes one amino acid from another and it may be neutral, acidic or basic.
Proteins are built from a set of 20 naturally occurring amino acids. Glycine is the simplest member because its R group is H, giving NH2CH2COOH.
Acidic and basic properties
The amino group can accept H+, whereas the carboxylic acid group can donate H+. Having both types of site makes an amino acid amphoteric.
| Group | Behaviour | Acid–base change |
|---|---|---|
| –NH2 | Basic | Accepts H+ |
| –COOH | Acidic | Can lose H+ |
| R group | Depends on its structure | Extra amino or carboxylic acid groups can also take part in acid–base reactions |
The two functional groups can also undergo reactions characteristic of amines and carboxylic acids. For example, the carboxylic acid group can undergo esterification, while amino groups can take part in the usual reactions of amines.
Naming amino acids
Individual amino acids are often identified using short conventional names or three-letter labels. When a systematic name is required from a given structure, the normal rules of IUPAC organic nomenclature can be applied.
The emphasis is therefore on being able to interpret and name a supplied structure rather than memorising the full set of conventional amino-acid names.
Exam Tip: When drawing an α-amino acid, make sure –NH2, –COOH, H and R are all bonded to the same carbon. For a naming question, work systematically from the usual IUPAC rules rather than relying on a memorised common name.